The present invention relates to a method for applying a plastic material onto at least one individual conductor of a high-temperature superconductor (HTS) cable in the form of a Roebel conductor, and to a high-temperature superconductor (HTS) cable produced by the method, the at least one individual conductor comprising at least one support and at least one superconducting layer.
Roebel conductors are often used in power engineering, since the “transposition” or “stranding” of the individual electrical conductors with one another leads to lower electrical losses when a current flows through the overall Roebel conductor. In a range of applications, the Roebel conductors are exposed to alternating magnetic fields. Coupling currents are in this case induced in the individual electrical conductors, which contribute to so-called AC losses. The coupling currents can be prevented by insulating the individual electrical conductors from one another.
In conventional Roebel cables, formed of copper or aluminum, conventional insulating varnishes are generally used for the insulation. Adaptation of the insulation method from conventional Roebel cables to high-temperature superconductor (HTS) cables in the form of a Roebel conductor, is possible only limitedly or not at all. On the one hand, high-temperature superconductor (HTS) cables generally comprise individual electrical conductors in tape form, the tapes having high aspect ratios. For example, typical aspect ratios of width to thickness in the case of Bi cuprate HTS tapes are greater than or equal to 10, and in the case of YBCO (yttrium barium copper oxide) tapes, greater than or equal to 20. For this reason, with conventional insulating varnishes, edge thinning or “dog bone” formation occurs, with a very inhomogeneous wall thickness of the insulation over the tape circumference.
On the other hand, the insulating materials must be usable at temperatures lower than 110 K without becoming brittle or frangible. Typical HTS materials only have their transition temperature, i.e. superconducting properties, beyond temperatures lower than 110 K. Only high-grade plastic materials do not become brittle or lose their mechanical integrity at these low temperatures. Only high-grade plastics can therefore be used for the insulation.
It should furthermore be noted that the wall thickness of the insulation reduces the so-called engineering current density, which represents the critical current or operating current divided by the line cross section. Small wall thicknesses of the insulation are therefore advantageous. In conventional varnish insulations, the wall thickness can only be limitedly minimized because of the edge thinning.
In the case of S-shaped transposition regions of individual Roebel conductors, insulation before forming the S-shape is possible only if the insulation has elastic properties. High-temperature superconductors are generally formed from ceramic materials, i.e. they are brittle. Only when the high-temperature superconductor allows plastic bending without breaking, as is the case for example with Bi cuprate HTS tapes, can insulation be carried out before forming the S-region. In the case of YBCO high-temperature superconductors, the S-region has to be formed by punching the individual conductor from a wider tape. Insulation is realistic only after the punching.
One simple insulation method involves single or double winding of the individual Roebel conductors with self-adhesive Kapton tape. The Kapton tape itself generally has a thickness of more than 13 μm. Owing to the adhesive layer and a necessary overlap, the added thickness per side of the individual conductor due to the winding is more than 30 μm. A winding technique with Kapton tape leads to reduced sliding properties of the individual conductors, in particular over one another. This leads to reduced flexibility or bendability of the cable formed from individual conductors. The winding technique is not usable for individual conductors with an S-region shape already formed.
Another insulation method is possible based on polyamide. In this case, the necessary overlap leads to a thickness increase due to the insulation of more than 50 μm per side of the individual conductor.
Alternatively, insulation can be carried out by coextrusion of PEEK (polyether ether ketone) in tube form. This is known, for example, from EP 1 273 015 B1. By this method, uniform insulation of the individual conductors is achieved even over the narrow sides of the HTS tapes. The thickness increase per side is from 20 to 40 μm. This technique has also proven suitable for HTS Roebel cables based on Bi cuprates. However, it is not suitable for insulating HTS Roebel cables based on YBCO material, in which the S-region formation has already taken place before the insulation.